Many types of polymerization processes are used in the preparation of synthetic polymers. For example, the polymerization of a monomer into a polymer can be conducted in a number of different types of reaction systems, including suspension polymerization systems, emulsion polymerization systems, solution polymerization systems, and bulk polymerization systems. Each of these systems has certain advantages and disadvantages.
In suspension polymerization systems, the initiator (catalyst) is dissolved in the monomer, the monomer is dispersed in water, and a dispersing agent is incorporated to stabilize the suspension formed. All suspension polymerization processes use some type of surfactant to keep the monomer globules dispersed during the reaction in order to avoid coalescence and agglomeration of the polymer. Not only does the suspension stabilizer affect the particle size and shape, but also the clarity, transparency, and film-forming properties of the resultant polymer. A variety of dispersing agents including water-insoluble, finely divided, inorganic materials and organic materials, depending upon the monomer to be polymerized, have been used as dispersing agents. Thus, for example, talc, barium, calcium, and magnesium carbonates, silicates, phosphates and sulfates, as well as poly(vinylalcohol), salts of styrene-maleic anhydride copolymers, vinyl acetate-maleic anhydride copolymers, starch, gelatin, pectin, alginates, methyl cellulose, carboxymethyl cellulose, bentonite, limestone and alumina have been used as suspending agents. A major advantage of suspension polymerization is that the polymeric products are obtained in the form of small beads which are easily filtered, washed, and dried. Water is a much more desirable diluent and heat-transfer medium than most organic solvents.
However, in certain polymerization processes, for example, the preparation of very high cis-1,4-polybutadiene, while utilizing nickel catalyst systems the presence of moisture is highly undesirable. Thus, suspension polymerization in a water medium is not an effective process for the synthesis of very high cis-1,4-polybutadiene utilizing nickel catalyst systems.
An emulsion polymerization process is considered to be a three-phase reaction system consisting of large droplets of the monomer, the aqueous phase containing the dissolved initiator and the colloidal particles of monomer-swollen polymer. While the emulsion polymerization process has the economic advantage of using water as the emulsion base, not all polymerization processes can tolerate the presence of water. Such is the case with the polymerization of butadiene into very high cis-1,4-polybutadiene using nickel catalyst systems. In order to recover dry polymers which are prepared by emulsion polymerization, it is, of course, necessary to coagulate the rubber from the latex. Coagulation is generally accomplished by adding a combination of salt and acid to the latex. This results in the formation of waste water which can present environmental problems.
In solution polymerization, an organic solvent is used which is capable of dissolving the monomer, the polymer, and the polymerization catalyst or initiator. Inasmuch as the polymer is soluble in the organic solvent which is used, there is a tendency for the viscosity of the solution to increase as the molecular weight of the polymer increases. If this continues over a period of time, the solution becomes too viscous to handle in conventional polymerization reaction systems unless the solids content is limited to a low level. In commercial polymerization processes, it is desirable to obtain a polymerization mass which has a high concentration of solid polymer and, at the same time, comprises a material which is easy to handle and does not agglomerate on the walls of the reaction vessel utilized. The polymeric solution is generally steam stripped in order to remove the solvent and unreacted monomer. The aqueous slurry of crumb rubber is usually pumped to a skimming tank, a water expeller and an extruder dryer in order to remove the water. The steam stripping and drying operations consume a large amount of expensive energy.
In nonaqueous dispersion polymerizations, an organic medium is utilized which is a very poor solvent for the polymer being produced. A dispersing agent is utilized in the organic medium in order to disperse the polymer being formed throughout the medium. The dispersing agents (dispersion stabilizers) which are utilized in such nonaqueous dispersion polymerizations are generally polymeric materials which can be block copolymers, random copolymers, or homopolymers. Nonaqueous dispersion polymerizations are described in detail in U.S. Pat. No. 4,098,980 and U.S. Pat. No. 4,452,960. Nonaqueous dispersion polymerization processes offer several distinct advantages over solution polymerizations and emulsion polymerizations including improved heat transfer, higher polymer concentrations in the reaction medium, increased production capacity, and energy savings.
Bulk polymerization is the direct conversion of liquid monomers to polymer. Such bulk polymerizations are generally carried out by the addition of an initiator to a simple homogeneous system containing one or more monomers. The polymers produced in such bulk polymerizations can be but are not necessarily soluble in their own monomers which are in effect utilized as the reaction medium. For example, polyisoprene is fairly soluble in isoprene and polypentadiene is fairly soluble in 1,3-pentadiene, but high cis-1,4-polybutadiene is not very soluble in 1,3-butadiene monomer. The synthesis of polystyrene by the addition of a free radical initiator to styrene monomer is a good example of a very common bulk polymerization. The principal advantage of a bulk polymerization process is that no solvent is utilized. Thus, the cost of solvent recovery and recycle is eliminated. One disadvantage of bulk polymerization reactions is that it is difficult to control the reaction temperature during polymerization. In fact, attempts to bulk polymerize many types of monomers have resulted in the reaction getting totally out of control. Due to this difficulty, bulk polymerization has not been widely utilized in the commercial preparation of synthetic rubbers.
The concept of preparing synthetic rubbers by bulk polymerization is not new. It has been known for many years that diene monomers can be polymerized into synthetic rubbers in the absence of a solvent. In fact, the Germans and Russians synthesized polybutadiene and polydimethylbutadiene in bulk during World War II using alkali metal catalysts in a batch process. French Pat. No. 8,702,167 discloses a process for the bulk polymerization of 1,3-butadiene monomer into high cis-1,4-polybutadiene. The process disclosed in French Pat. No. 8,702,167 more specifically involves:
(1) charging into a reaction zone the 1,3-butadiene: a catalyst system comprising (a) an organoaluminum compound, (b) a soluble nickel containing compound, and (c) a fluorine containing compound;
(2) allowing the 1,3-butadiene to polymerize into high cis-1,4-polybutadiene to a conversion of at least about 60 percent while utilizing conditions under which there is sufficient evaporative cooling in said reaction zone to maintain a temperature within the range of 10.degree. C. to 130.degree. C.: and
(3) continuously withdrawing said high cis-1,4-polybutadiene from said reaction zone. In order to reduce the molecular weight of the high cis-1,4polybutadiene, the polymerizations of French Pat. No. 8,702,167 can be conducted in the presence of at least one molecular weight regulator selected from the group consisting of .alpha.-olefins, cis-2-butene, trans-2-butene, allene, 1,4-pentadiene, 1,5-hexadiene, 1,6-heptadiene, 1,2,4-trivinylcyclohexene, 4-vinyl-1-cyclohexene, 1-trans-4-hexadiene, and hydrogen. The reaction zone utilized in such techniques can be a self-cleaning extruder-reactor.
The possibility of preparing synthetic rubbers through bulk polymerization is an attractive possibility. This is because it would eliminate the need for utilizing solvents which must be separated from the rubber and recycled or otherwise disposed of. The cost of recovery and recycle of solvent adds greatly to the cost of the rubber being produced and can cause certain environmental problems. Recovery and separation of the rubber from the solvent also requires additional treatment and equipment, all of which further increase the cost of the rubber. The purification of solvents being recycled can also be very expensive and there is always the danger that the solvent may still retain impurities which will poison the polymerization catalyst. For example, benzene and toluene can form arene complexes with the catalyst which inhibit polymerization rates and which can result in the formation of polymers having lower molecular weights.
Even though bulk polymerization offers many substantial advantages, it cannot be utilized in conjunction with lithium catalyst systems. In other words, a satisfactory means for synthesizing synthetic rubbers with lithium catalysts is not available. Heretofore, synthetic rubbers prepared with lithium catalyst systems have been made by solution polymerization. Unfortunately, such solution polymerizations require the use of large amounts of organic solvent. This is because it is extremely difficult to prepare such synthetic rubbers on a commercial basis at solids contents of greater than about 20%. In commercial solution polymerization techniques solids contents within the range of 15% to 18% are typically employed. Even though numerous attempts have been made to reduce the amount of organic solvent required in such solution polymerizations, all such attempts have heretofore been unsuccessful.